Giant Magnetoresistance Behavior Research Articles

Unusual hysteresis and giant low-field magnetoresistance in polycrystalline sample with nominal composition ofLa2/3Ca1/3Mn0.955Cu0.045O3

Unusual thermal and magnetic hysteresis is observed near the insulator-metal transition of ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{3}(x=4.5%).$ For the temperature range at which the hysteresis appears, the sample shows unusual giant magnetoresistance (MR) behavior even for a low field of $\ensuremath{\sim}0.3\mathrm{T}.$ The maximum MR has a value of $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}(H=0)$ as high as $\ensuremath{\sim}90%$ for the 0.3 T field. A possible discussion is presented by considering the sample as a granular system consisting of manganese grains surrounded by some surface layer created due to the Cu segregation towards the grain surface.

Preparation, Structure, Magnetic, and Magnetotransport Properties of Electrodeposited Co ( Ru ) / Ru Multilayers

It is demonstrated that Co(Ru)/Ru multilayers with a hexagonal close-packed structure can be produced by electrodeposition. For appropriate layer thicknesses, clear giant magnetoresistance (GMR) behavior has been found. The observed GMR is very small (less than 0.1% for 8 kOe at room temperature) but not smaller than the GMR of Co/Ru multilayers prepared previously by high-vacuum methods. It is argued that neither structural imperfection of the multilayers nor chemical intermixing at the interfaces can be the main reason for the small GMR observed in the Co-Ru system for any preparation technique. It is suggested that the strongly reduced spin-dependent scattering characteristics of the interfacial Co-Ru alloys, which was described in a previous paper, might explain the low GMR of Co/Ru multilayers. © 2002 The Electrochemical Society. All rights reserved.

Transition from negative magnetoresistance behavior to positive behavior in Co20(Cu1−xGex)80 ribbons

We report a transition of the giant magnetoresistance (GMR) behavior in nanocrystalline Co20(Cu1−xGex)80 ribbons from negative to positive, as the semiconductor Ge substitutes for the Cu matrix. The growth of the hexagonal Co3Ge2 compound leads to a change of the physical origin of the GMR. The normal spin-dependent transport behavior in the CoCu granular system evolves into Coulomb blockade behavior of electronic tunneling in ribbons with a Co/Co3Ge2/Co junctionlike configuration.

Giant Magnetoresistance of Electrodeposited Ni81Cu19 / Cu Multilayers

The room-temperature magnetoresistance (MR) characteristics were investigated for electrodeposited multilayers as a function of the constituent magnetic and nonmagnetic layer thicknesses. The maximum giant magnetoresistance (GMR) was obtained around 3 nm magnetic layer thickness and 1 nm nonmagnetic layer thickness. For multilayers with GMR behavior, especially around the optimum sublayer thickness combination, the MR curves could be considered as consisting of a low-field and a high-field contribution. The latter contribution can be ascribed to chemical intermixing at the interfaces between the magnetic and nonmagnetic layers due to an exchange reaction between Cu and Ni. We have identified two factors that may significantly influence the GMR of electrodeposited multilayers: the position and orientation of the investigated sample section on the cathode surface during deposition and the deterioration of the particular citrate/sulfate bath used. © 2002 The Electrochemical Society. All rights reserved.

The Limit of the Giant Magnetoresistance Effect in Only Three Layers

The giant magnetoresistance (GMR) behavior of the very simplest structures consisting of one nonmagnetic layer sandwiched in between two ferromagnetic layers has been thoroughly investigated. It has been shown that these trilayers exhibit room temperature GMR effect amplitudes of 16% that match the GMR values of well designed biased spin-valve structures. The oscillatory interlayer exchange coupling (IEC) has been determined for different series of structures like glass/{Py(= Ni 80 Fe 20 )/FM}/spacer/FM, with FM = Co or Ni 57 Co 43 and the spacer materials such as Cu or Cu 85 Ag 10 Au 5 , over a wide range of spacer layer thickness. The GMR effect amplitude has also been extracted as a function of the individual layer thickness. It has been demonstrated, that an extended Stoner-Wohlfarth model can very well be used to predict the GMR characteristics of these trilayers or more complicated structures. The sensitivity of these structures can be as high as 40%/mT. GMR effects of up to 20% at room temperature have been realized with dual structures of type glass/(Py/Co}/Cu/Co/Cu/Co. Changing the thickness of the middle Co layer the GMR effect amplitude as well as the saturation field can be tailored. In addition it has been shown, that the Cu 85 Ag 10 Au 5 spacer material is much more stable to temperature exposure than pure Cu.

Magnetic and transport behavior of single-crystallineDy2PdSi3

The results of magnetization, magnetic susceptibility $(\ensuremath{\chi}),$ electrical measurements, and magnetoresistance measurements as a function of temperature (2--300 K) in the single-crystalline ${\mathrm{Dy}}_{2}{\mathrm{PdSi}}_{3}$ are reported for two different directions ([0001] and $[101\ifmmode\bar\else\textasciimacron\fi{}0])$ of the crystal. The results reveal that there is an onset of magnetic ordering below 8 K. The high-temperature paramagnetic Curie temperature is positive for both directions, though antiferromagnetic coupling dominates in the basal plane; the magnetic coupling along the c axis is of a ferromagnetic type. The electrical resistance data for both the directions indicate the formation of magnetic Brillouin-zone boundary gaps. A finding of importance is that the broad peak in the temperature dependent $\ensuremath{\chi}$ around 50 K noted for the analogous Tb compound is not observed in the present investigations, though there is a weak difference between the zero-field-cooled and field-cooled $\ensuremath{\chi}$ versus T curves below 40 K (the implications of which are not clear at present). The results establish that this compound exhibits giant magnetoresistance behavior at low temperatures. We also compare the observed anisotropic behavior with those in Tb and Gd compounds.

From direct to inverse GMR: introduction of Cr in Fe/Cu superlattices

It is well known that layered systems can present giant magnetoresistance (GMR). Particularly, the multilayers Fe N /Cu 8/Fe M /Cr/ Fe M /Cu 8 show inverse GMR. That is, the electrical conductivity decreases with the applied magnetic field. In the most common multilayers the conductivity increases with the applied field and this kind of GMR is called direct. In general the GMR is attributed to spin-dependent scattering in the bulk and mainly at interfaces. In this work we calculate the electronic band contribution to the GMR for Fe 3/Cu 4/Fe/Cr/Fe/Cu 4 and Fe 3/Cu 4 multilayers within the semiclassical approximation. The electrical conductivity is obtained in the ballistic and diffusive regimes. The results show a large change in the GMR behavior when one layer of Cr is introduced within the Fe layers. The GMR calculated in the CPP configuration (current flowing perpendicular to layers) of Fe 3/Cu 4 is of the direct type, with a value of about 40% while that obtained for Fe 3/Cu 4/Fe/Cr/Fe/Cu 4 is inverse and of the order of 45%. In the CIP configuration (current flowing parallel to the layers) the calculated GMR is direct with a value of about 35% for the system without Cr while, by the introduction of Cr, we obtain also a direct GMR but of about 3%.

Microstructure and Giant Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers

Direct current plating, pulse plating, two-pulse plating, and reverse pulse plating were used to produce electrodeposited Co-Cu alloys and Co-Cu/Cu multilayers under galvanostatic control from an electrolyte containing and Atomic force microscopy, X-ray diffraction, and transmission electron microscopy were used to study the sample structure and morphology. Direct current plating resulted in a alloy with nearly equal amounts of face-centered cubic (fcc) and hexagonal close packed phases, while all pulsed current methods yielded multilayers with fcc structure. Giant magnetoresistance (GMR) behavior was observed in the multilayers with a maximum magnetoresistance (MR) ratio of about 9% as measured at 8 kOe. The shape of the MR curves and the magnitude of the GMR were very similar, regardless of the sign of the current between the Co deposition pulses. The results of structural studies also confirmed the formation of a multilayer structure for each pulsed electrodeposition mode. The conclusion was that the spontaneous exchange reaction between Co and is responsible for the formation of a pure Cu layer even under reverse pulse plating conditions. The GMR of the multilayer deposits decreased with increasing bilayer number, due to the deterioration of the microstructure as the deposit grew. © 2001 The Electrochemical Society. All rights reserved.

Structure and Giant Magneto-Resistive Properties of Co and CoFe nano-particles in a Au matrix

ABSTRACTThis paper focuses on the relation between the microstructure and the Giant Magneto-Resistive (GMR) properties of Au80Co20 and Au80Co10Fe10 granular alloys. After annealing of quenched samples large differences in structure of Co and CoFe nano-particles arise: e.g. fcc Co versus bcc CoFe, spinodal decomposition of Co versus nucleation and growth of FeCo, truncated octahedrons for Co and plates for CoFe. Particularly the shape of the precipitates and the state of coherency at the interface between magnetic particle and non-magnetic matrix turned out to have important influences on the GMR behavior.

Structural and magnetoresistance characteristics of CoFe/Ag/NiFe/Ag composite discontinuous multilayers

The structural, magnetic, and giant magnetoresistance (GMR) behaviors of new types of composite multilayers were investigated. The main objective of this study was to accomplish high MR sensitivity while maintaining magnetic softness. The MR sensitivity of CoFe/Ag/NiFe/Ag multilayers was higher compared to CoFe/Ag multilayers mainly due to the switching field difference between NiFe and CoFe. The x-ray diffraction analysis and temperature dependence of Δρ provided evidence of the structural evolution in the CoFe/Ag/NiFe/Ag upon annealing. The temperature dependence of Δρ in CoFe/Ag/NiFe/Ag multilayers annealed for 1 min compared with as-deposited multilayers were reduced because the interface structure was sharpened and structural defects were reduced.

Influence of Si buffer layer on the giant magnetoresistance effect in Co/Cu/Co sandwiches

The Co/Cu/Co sandwiches with a semiconductor Si buffer layer were prepared by high vacuum electron-beam evaporation. The influence of the Si buffer layer with different thickness on the giant magnetoresistance (GMR) effect in the Co/Cu/Co sandwiches was investigated. It was found that the GMR showed an obvious anisotropy when the thickness of Si buffer layer was larger than or equal to 0.9 nm, and that the GMR was basically isotropic with an Si buffer layer thinner than, 0.9 nm. The anisotropic behavior of GMR can be ascribed to the in-plane magnetic anisotropy in the sandwiches. Due to the interdiffusion at the Si buffer/Co interface, a Co2Si interface layer with a good (301) texture formed and induced the in-plane magnetic anisotropy in the sandwiches. The dependence of the crystalline texture of the sandwiches on the thickness of Si buffer layer was also studied.

Influence of Fe and Co additions on magnetization and magnetoresistivity of MnAu2 melt-spun alloys

Melt-spun MnAu2 ribbons modified by ternary magnetic additions (Fe and Co) show a large giant magnetoresistance (GMR) effect up to a magnetoresistance ratio MR={[R(H=7T)−R(0)/R(0)]} of the order of −10% at room temperature. Pure MnAu2 samples show a transition from positive magnetoresistance at low temperatures (e.g., MR=+23% at 10 K and μ0H=5 T) to a GMR behavior with values of about −8% at 295 K and 5 T. At 135 K the resistivity is nearly independent of the applied magnetic field. The strong increase of |MR| at room temperature is only found with applied fields larger than a threshold field, where a metamagnetic transition occurs. Alloying the compound with Co or Fe this threshold field can be reduced or suppressed. This effect is due to softening of the antiferromagnetic behavior of the (Mn; Co or Fe)Au2 samples. For an alloy with 5 at. % Fe as substitutes for Mn, no metamagnetic transition is observed in the temperature range between 10 and 300 K. For both 5 at. % Co or Fe additions a GMR behavior is observed in this temperature range with higher sensitivity at low fields, e.g., (Mn0.95Fe0.05)Au2 has values of MR=−12% and −1% in a field of 1 T at 10 and 295 K, respectively. At low temperatures, an anomalous increase of resistance at higher fields (above 1–2 T) is observed for these alloys.

The GMR effect in annealed Co/Co2TiSn thin films

The giant magnetoresistance (GMR) effect of Co/Co2TiSn thin films with two magnet phases was observed in samples annealed in vacuum and N2(10% H2) gas atmospheres. The perpendicular magnetization curve of the films have the hysteresis loop of a typical bubble domain material. Various features of electrical resistivity at room temperature of the annealed films suggest GMR behavior. The resistivity decreases with increasing field and saturates at a field of around 2 kOe which is the same as in-plane saturation field from magnetization measurements. In the film annealed in N2 gas, the magnetoresistivity, δρ(H), is 1.43×10−7 Ω cm and the GMR is 0.18%. In the film annealed in vacuum, the δρ(H) and GMR are about 3.3×10−8 Ωcm and 0.12%, respectively. The GMR effect in annealed Co/Co2TiSn films indicates the scattering of spin polarized conduction electrons because of the antiparallel exchange coupling at the phase boundary between the two magnetic phases. The scattering process of conduction electrons at the phase boundary was modeled in relation to the magnetization process.

CoFe–Cu granular alloys: From noninteracting particles to magnetic percolation

CoFe–Cu granular films with ferromagnetic content ranging from 0.10 to 0.33 by volume were prepared by radio frequency sputtering. As-cast samples were rapidly annealed at various temperatures up to 750 °C to promote the segregation of CoFe particles within the metallic matrix. Magnetic and transport properties suggested that this family of samples may be classified into three groups: (i) below about 0.20 volume content of CoFe, all samples display the typical features of a granular solid constituted by a random distribution of nanometric CoFe particles within a Cu matrix, and the maximum magnetoresistance is about 20% at low temperature (giant magnetoresistance); (ii) for as-cast samples within 0.20 and 0.30 of volume concentration, magnetoresistance and magnetization display complex bimodal behavior and large metastable effects associated with the interparticle interactions, which stabilize a domain-like microstructure well below the volume percolation threshold (0.55), as already observed in CoFe–Ag(Cu) granular alloys. As a consequence of the large magnetic correlations, magnetoresistance is very low (1%–3%). Through annealing, the microstructure and therefore the transport properties evolve to those of a classical giant magnetoresistance system with large particles; and (iii) above about 0.30 of volume content (and still below the volume percolation threshold), as-cast samples display both anisotropic and giant magnetoresistance, as also observed in other granular alloys. Annealing leads to complete segregation and to the formation of large magnetic particles, which results in a transition from mixed behavior of both anisotropic and giant magnetoresistance (GMR) regimes to a giant magnetoresistance regime, with a maximum GMR of about 7%.

Geometrically induced giant magnetoresistance behavior in quarter-micrometer strip pattern

The present article reports a geometrically induced magnetoresistance (MR) behavior observed in magnetic strips consisting of a symmetric layer structure with lateral sizes down to 0.2 μm. The MR ratio was markedly improved from 2.8% for an as-grown film of Co(6 nm)/Cu(6 nm)/Co(6 nm) to 4.6% by structuring it into a strip with 0.3 μm width. The parabolic MR behavior observed in the hard-axis direction suggests that the magnetization in the two magnetic layers rotates toward the opposite directions, which can be explained by the magnetostatic interaction between the surface magnetic charges at the pattern edges. The dependence of MR saturation field on the pattern width agrees well with a theoretical prediction based on a coherent rotation model. The MR ratio was markedly increased by the increase of the layer number and the thickness of the magnetic layer, the highest value of 7.8% was obtained for a strip made of [Co(6 nm)/Cu(4 nm)]2Co(6 nm). A linear- and hysteresis-free MR response was performed with an alternative field modulation under a transverse bias field.

Effect of biquadratic exchange coupling on magnetoresistance and magnetization process in magnetic bilayer systems

An improved model is proposed to deal with the magnetic bilayer systems taking into account the contribution of the anisotropy energy and biquadratic exchange coupling to elaborate on the evolution of the magnetoresistance (MR) ratio and magnetization process. The results indicate that the characteristic behavior of the MR ratio depends distinctly on both the biquadratic coupling constant and the layer thickness. The profile of the MR ratio was found to vary from an inverted bell shape to a concave pyramid with increasing biquadratic coupling strength, and decays sharply with the layer thickness. This model calculation helps us to provide a venue for further understanding the MR or giant magnetoresistance behavior of the magnetic multilayer system.

Large change of the giant magnetoresistance behavior after changing the spacer from Cu to CuMn alloy in Co/spacer/Co structures

The giant magnetoresistance (GMR) effects in sandwiched Co/Cu/Co and Co/CuMn/Co structures have been investigated. The GMR oscillates with the spacer thickness for both cases, but is nearly antiphased. With diluted Mn atoms in the Cu spacer, the GMR curve as a function of the magnetic field changes a lot, and the saturation/switching field for GMR can be reduced greatly compared with that in Co/Cu/Co systems. This may indicate one way to obtain a highly sensitive GMR.

Effect of annealing and chemical composition on the giant magnetoresistance of electron beam deposited Co xCu (100− x) (11⩽ x⩽45) granular films

A series of granular thin films of compositions Co x Cu 100− x (11⩽ x⩽45) and 150 nm thickness are deposited on mica and glass substrates by the electron beam evaporation technique. Structural and morphological investigations are made by X-ray diffraction, electron diffraction and transmission electron microscopic techniques. Films are nanocrystalline and the crystallite size increases upon annealing at 400°C/0.5 h in vacuum. The magnetization and magnetoresistance of the films are measured. Films with cobalt concentration lower than 13 at% show superparamagnetic and giant magnetoresistance behavior and for Co concentration higher than 30 at% show anisotropic magnetoresistive behavior. Annealed films of compositions (17⩽ x⩽25) show maximum GMR of 5–6% in a magnetic field of 9 kOe.

Giant magnetoresistance behavior of granular Fe and Co implanted Ag thin films

The magnetoresistive behavior of granular thin films prepared by Fe and Co implantation in Ag thin films is reported. Ag thin films (∼2000 Å) were deposited by evaporation or laser ablation onto Si and SiO2 substrates and implanted with Fe or Co at fluences up to 1017 at. %/cm2. The magnetoresistive response obtained after implantation was found to increase with the implanted fluence. An increase of the magnetoresistive response by a factor of 3–4 can be achieved after annealing the films in a conventional furnace at 620 K under vacuum. The best value of magnetoresistance obtained so far is 9% at 10 K and 1.5% at room temperature for a film implanted with Co at a fluence of 8×1016 at. %/cm2 and annealed at 620 K.

ALaMn2O6-y (A = K, Rb): Novel Ferromagnetic Manganites Exhibiting Negative Giant Magnetoresistance

We report the synthesis of a new series of manganese oxides, ALaMn(2)O(6-y), for A = K, nb, and Cs, that adopt an ordered superstructure of the perovskite. The Ii and Rb phases are tetragonal (a(t) similar to 7.75, c(t) similar to 7.10 Angstrom) and the Cs phase is orthorhombic (a(0) = 5.511, b(0) = 5.468, c = 7.311 Angstrom) indicating 2a(p) x 2a(p) and root 2a(p) x root 2a(p) x 2a(p) (a(p) = perovskite subcell) superstructures, respectively. The K and Rb phases are ferromagnetic (T-c similar to 327 and 300 K) metals, exhibiting a giant magnetoresistance behavior similar to alkaline earth metal substituted lanthanide manganites. The results reveal that it is possible to induce Mn(III)I Mn(TV) mixed valency and the related electronic properties through action deficiency in ordered perovskites of the kind ALaMn(2)O(6).